We have shown that the pineal gland of the chicken can be maintained in vitro for at least 5 days. Isolated glands continuously superfused with culture medium manufacture the putative hormone melatonin and release it into the medium. Timed collection of medium samples reveals that the isolated organ cultured gland retains a circadian rhythm of melatonin output. Further, the in vitro melatonin rhythm can be influenced by light. The chicken pineal gland must therefore contain photoreceptors, oscillator(s) and melatonin synthetic machinery. Using radioimmunoassay techniques to measure the circadian rhythms of melatonin released into the medium by cultured pineals we propose to investigate: (1) the nature of the pineal photoreceptor; we will obtain an action spectrum and study the effect of Vitamin A deprivation; (2) the effects of light on the pineal pacemaker; we will characterize entrainment by light cycles, phase shifting by light pulses and period changes produced in the circadian oscillator by constant light; (3) the pathway that couples the photoreceptor with the circadian oscillator: by manipulating the ionic composition of the medium in ways that produce known biophysical effects we will attempt to block light induced phase shifts of the rhythm and by applying putative neurotransmitters we will attempt to mimic light induced phase shifts; (4) whether the pineal contains more than 1 circadian oscillator; we will divide single pineals into several pieces and assay each piece for circadian rhythmicity; (5) the possible involvement of cyclic nucleotides with generation of circadian oscillations; we will attempt to influence the period and phase of the circadian oscillator with phosphodiesterase inhibitors and cyclic nucleotide analogues. The avian pineal is the only vertebrate circadian oscillator that it has so far been possible to study in vitro. It is thus the only model system from which we can presently hope to learn about the internal mechanisms of vertebrate circadian oscillators.